What causes severe muscle loss and skin to be flabby?

If they are in end stage COPD it could very likely be cachexia .

Look up "reasons skin ages" and "causes of loss of muscle tone". Also use your search engine to look up "COPD and muscle tone" and "COPD and skin aging". Remember, in questions like this the internet with its wealth of information, is your friend. You might also look up "aging and loss of muscle tone" and "aging and skin elastin". Do know also that the aging of skin has cultural and race connections. Certain populaces have more problem with aging skin. As to muscle tone, often an inability to stay active and exercise has profound effects on muscle and bone. We are meant to MOVE. Once we stop the trouble begins as far as deterioration is concerned. You can include any and everything in this, including our bones which will actually stop manufacturing needed components for survival without movement.

Read up on anatomy. It is a fascination that is endless.

Look up cachexia in COPD

Please provide more information other than this person has COPD. Lots of health issues can cause the very generic symptoms you described...and we're not doctors. If these symptoms are of concern, I would take this person to their primary doc first. Then maybe discuss with the pulmonologist.